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Geosyntec consultants 2.3.5 Groundwater 2.3.5.1 Depth The groundwater elevations at RRI generally range from about 1 to 12 feet below msl (0 to 15 feet bgs) and fluctuate by about 2 to 5 feet between the dry and wet seasons (Appendix D). 2.3.5.2 Flow The primary factors that control the groundwater flow system at RRI are recharge to groundwater from the rivers and channels that surround RRI and discharge of shallow groundwater into the island's internal drainage channel system (Figure 13). The hydraulic influence of the surrounding surface water bodies, combined with the discharge to the surface drainage channel system, produces a general flow pattern toward the interior of the island (Figure 20). The levees around the perimeter of RRI maintain surface water elevations higher than the RRI topography. Along the perimeter of RRI, groundwater flows from the higher elevation surface water bodies (1 to 10 feet above msl) towards the lower elevation groundwater underlying RRI (0 to 12 feet below msl). The Stockton Deep Water Channel is up to 29 feet deeper than Burns Cutoff and has a larger influence on RRI groundwater than Burns Cutoff or the San Joaquin River because it intersects a much larger cross-section of the water-bearing zone (Tetra Tech, 2002). Surface water elevations in all three surface water bodies should be almost identical at any given time. Figure 21 shows a comparison of groundwater elevations in monitoring wells along the perimeter of RRI to surface water elevations measured at USGS stream gauge#11304810 in the San Joaquin River. The surface water elevations have been higher during both high and low tide over time (1 to 10 feet above msl)than the groundwater elevations measured in the monitoring wells at RRI over time (0 to 12 feet below msl). This indicates that recharge from the surface water bodies and inward gradients towards the interior of RRI are continuously maintained (Geosyntec, 2018a; Geosyntec, 2020d; Geosyntec, 2021b). While the magnitude of the groundwater gradients may be tidally influenced, the direction of the groundwater flow is continuously inwards from the surrounding rivers (higher elevation)towards the interior of RRI (lower elevation), and there is no tidal influence on the groundwater flow direction. Additionally, artesian conditions have not been encountered in the borings drilled to 80 to 90 feet bgs at RRI, indicating the groundwater elevations in the water-bearing zone above the aquitard are lower than the water elevations in the surrounding rivers (Geosyntec, 2020c; Geosyntec, 2020d). The Port has not drilled borings below the aquitard and the Navy retains responsibility for groundwater below 90 feet bgs per the ESCA. Figure 20 shows the general estimated potentiometric surface at RRI. The mean surface water elevation during the third quarter 2020 from stream gauge 11304810 was used as the average groundwater elevation at the surface water and groundwater interface around the perimeter of RRI. Groundwater elevations were calculated based on depth to water measurements taken during the third quarter 2020 and the surveyed top of casing elevations. The 2018 top of casing elevations were used to calculate the groundwater elevation. The groundwater elevations from the third quarter 2020 were used to generate the potentiometric surface contours. The third quarter 2020 groundwater monitoring event was used as representative because the Landfill Area wells were not gauged in 2021, in accordance with the third addendum to the 2018 GMWP. RRI Groundwater Monitoring Work Plan 13 June 2022